NOVBMBEE 6, 1896.] 



SCIENCE. 



671 



tory whicli must have been passed through 

 before the ancestor of one of the higher of 

 these nine Phyla came into being. Let us 

 consider one or two examples, since the es- 

 tablishment of this position is of the utmost 

 importance for our argument. First con- 

 sider the past history of the Vertebrata — of 

 the common ancestor of our Balanoglossus, 

 Tunicates, Amphioxus, Lampreys, Fishes, 

 Dipnoi, Amphibia, Eeptiles, Birds and Mam- 

 mals. Although zoologists differ very widely 

 in their opinions as to the af&nities of this 

 ancestral form, they all agree in maintain- 

 ing that it did not arise direct from the Ne- 

 matophora in the lower sub-grade of Metazoa, 

 but that it was the product of a long his- 

 tory within the Coelomate sub-grade. The 

 question as to which of the other Coelomate 

 Phyla it was associated with will form the 

 subject of one of our discussions at this 

 meeting; and I will, therefore, say no more 

 upon this period of its evolution, except to 

 point out that the very question itself, ' the 

 ancestry of Vertebrates,' only means a rela- 

 tively small part of the evolutionary his- 

 tory of the Vertebrate ancestor within the 

 Coelomate group. For when we have de- 

 cided the question of the other Coelomate 

 Phylum or Phyla to which the ancestral 

 Vertebrate belonged, there remains, of 

 course, the history of that Phylum or those 

 Phyla earlier than the point at which the 

 Vertebrate diverged, right back to the 

 origin of the Coelomata ; while, beyond and 

 below, the wide gulf between this and the 

 Ooelentera had to be crossed, and then, 

 probably after a long history as a 

 Coelenterate, the widest and most signifi- 

 cant of all the morphological intervals — 

 that between the lowest Metazoon and the 

 highest Protozoon — was traversed. But 

 this was by no means all. There remains 

 the history within the higher Protozoan 

 sub-grade, in the interval from this to the 

 lower, and within the lower sub-grade it- 

 self, until we finally retrace our steps to 



the lowest and simplest forms. It is impossi- 

 ble to suppose that all this history of change 

 can have been otherwise than immensely 

 prolonged ; for it will be shown below that 

 the only evidence which is available sup- 

 ports the belief that the changes during 

 these earlier phases were at least as slow as 

 those which occurred later. 



If we take the history of another of the 

 higher Phyla, the Appendiculata, we find 

 that the evidence points in the same direc- 

 tion. The common ancestor of our Eoti- 

 fera, earthworms, leeches, Peripatus, cen- 

 tipedes, insects, Crustacea, spiders and scor- 

 pions, and forms allied to all these, is gen- 

 erally admitted to have been Chsetopod-like, 

 and probably arose in relation to the be- 

 ginnings of certain other Coelomate Phyla, 

 such as the Gephyrea and perhaps Mollusca. 

 At the origin of the Coelomate sub-grade 

 the common ancestor of all Coelomate Phyla 

 is reached, and its evolution has been 

 already traced in the case of the Vertebrata. 



What is likely to be the relation between 

 the time required for the evolution of the 

 ancestor of a Coelomate Phylum and that re- 

 quired for the evolution, which subsequently 

 occurred, within the Phylum itself? The 

 answer to this question depends mainly 

 upon the rate of evolution in the lower 

 parts of the animal kingdom as compared 

 with that in the higher. Contrary, per- 

 haps, to anticipation, we find that all 

 evidences of rapid evolution are confined to 

 the most advanced of the smaller groups 

 within the highest Phyla, and especially to 

 the higher Classes of Vertebrata. Such 

 evidence as we have strongly indicates the 

 most remarkable persistence of the lower 

 animal types. Thus in the Class Imper- 

 forata of the Reticularia (Foraminifera) 

 one of our existing genera (Saccamina) 

 occurs in the Carboniferous strata, another 

 {Trochammina) in the Permian, while a 

 single new genus (^ReceptaeuUtes) occurs in 

 the Silurian and Devonian. The evidence 



